Transmyocardial Revascularization
In the treatment of heart disease, one method of improving myocardial blood supply is called transmyocardial revascularization (TMR), the creation of channels in the myocadium of the hear. The procedure using needles in a form of surgical "myocardial acupuncture" has been used clinically since the 1960s. Deckelbaum. L. I., Cardiovascular Applications of Laser Technology, Lasers in Surgery and Medicine 15:315-341 (1994). The technique relieves ischermia by allowing blood to pass from the ventricle through the channels either directly into other vessels communicating with the channels or into myocardial sinusoids which connect to the myocardial microcirculation.
Numerous surgical TMR studies have been performed, including early studies using needles to perform myocardial acupuncture, or boring, to mechanically displace and/or remove tissue. Such studies have involved surgically exposing the heart and sequentially inserting needles to form a number of channels through the epicardium, myocardium, and endocardium to allow blood from the ventricle to perfuse the channels. The early studies using needles showed that the newly created channels were subject to acute thrombosis followed by organization and fibrosis of clots resulting in channel closure. Interest in TMR using needles waned with the knowledge that such channels did not remain open. However, interest in TMR procedures recurred with the advent of medical lasers used to create TMR channels. Histological evidence of patent, endothelium-lined tracts within laser-created channels shows that the lumen of laser channels can become hemocompatible and resists occlusion. Additionally, recent histological evidence shows probable new vessel formation adjacent collagen occluded transmyocardial channels, thereby suggesting benefits from TMR with or without the formation of channels which remain patent.
Surgical TMR procedures using laser energy have been described in the prior art. U.S. Pat. No. 4,658,817 issued Apr. 21, 1987 to Hardy teaches a method and apparatus for surgical TMR using a C0.sub.2 laser connected to an articulated arm having a handpiece attached thereto. The handpiece emits laser energy from a single aperture and is moved around the epicardial surface of the heart to create the desired number of channels. U.S. Pat. No. 5,380,316 issued Jan. 10, 1995 to Aita et al. purports to teach the use of a flexible lasing apparatus which is inserted into the open chest cavity in a surgical procedure. A lens at the distal end of the flexible apparatus is used to focus laser energy, and the apparatus is moved about the epicardial surface of the heart to create the desired number of channels.
Since TMR involves creating channels through the endocardium into the lower left chamber of the heart, it is also desirable to create TMR channels percutaneously, i.e., by extending a catheter apparatus through the vasculature into the ventricle and creating the channels through endocardial surfices and into myocardium. Performing such percutaneous TMR is desirable for a number of reasons. Percutaneous catheter procedures are typically less traumatic to the patient compared to surgical procedures. Adhesions between the pericardial sac and epicardium are eliminated. Percutaneous TMR with a catheter apparatus also offers an alternative solution to persons who are not candidates for surgical procedures.
TMR procedures generally involve creating a plurality of channels within the myocardium. In performing the procedure, particularly percutaneously, it is desirable to have information relating to the depth of channels created, placement of the channels relative to the heart walls and wall thickness of the beating heart. None of the TMR or atherosclerosis devices described above or elsewhere provide such information.
Ultrasound
Ultrasound systems are widely used in medical applications. Sound waves above the frequency normally detectable by the human ear, that is, about 16 to 20 kHz are referred to as ultrasonic waves.
U.S. Pat. No. 4,576,177 issued Mar. 18, 1986 to Webster, Jr. teaches a catheter for removing arteriosclerotic plaque. The apparatus comprises a catheter having an optical fiber for transmitting laser energy and an ultrasound transducer. One embodiment of the device is operated in two different modes--a pulse-echo mode and a pulsed-Doppler mode. In the pulse-echo mode an electrical impulse delivered to the transducer transmits an ultrasound pulse, returning echoes thereof causing electrical "signature" signals. In the pulsed-Doppler mode, ultrasonic echoes from tone bursts generated in response to electrical bursts transmitted to the ultrasound transducer are used to determine the blood flow velocity at two selected distances from the catheter tip. The tissue signature and the change in blood flow velocity are used to determine the presence of occlusions in blood vessels.
U.S. Pat. No. 4,658,827 issued Apr. 21, 1987 to He et al teaches an ultrasound scanner for tissue characterization. A method and system are disclosed for simultaneously obtaining accurate estimates of the attenuation coefficient of the tissue and an index describing the heterogeneity of the scatterers within the tissue. According to the invention, there is provided a method and apparatus for tissue characterization by transmitting ultrasound energy into the sample tissue, and receiving and processing return echo signals.
U.S. Pat. No. 4,672,963 issued Jun. 16, 1987 to Barken teaches an apparatus and method for computer controlled laser surgery using an ultrasound imaging system. The position of the laser energy delivery device is monitored with an ultrasound probe. The probe, in conjunction with a computer system, provides a multiplicity of cross-section images of the portion of body tissue within the range of emitted destructive radiation.
U.S. Pat. No. 5,109,859 issued May 5, 1992 to Jenkins teaches an ultrasound guided laser angioplasty system. This system is also directed to the application of removal of atherosclerotic plaque from coronary arteries of patients with heart disease. A probe with a phased-array ultrasound transducer will produce images of vascular tissue acquired in a plane that is 30.degree. forward of the tip of the catheter to prevent vascular perforation. As above, the catheter provides primarily lateral imaging.
U.S. Pat. No. 5,158,085 issued Oct. 27, 1992 to Belikan et al. teaches a lithotripsy ultrasound locating device using both a locating and a therapy transducer in a fixed relationship. One or more locating ultrasound transducers, each axially rotatable and extendable, generate a signal representing the distance between the locating transducer and the focus of the second transducer, used to transmit therapeutic amounts of ultrasound for fragmentation of a concretion. The locating transducers have two or more crystal rings, thus having two or more focal ranges, and operate according to annular phased-array principles.
U.S. Pat. No. 5,313,950 issued May 24, 1994 to Ishikawa et al. teaches another ultrasound probe. A rotor moves and/or rotates a piezoelectric transducer and/or a reflector and is driven by a stator outside the object under examination. Both forward as well as lateral firing of ultrasound is taught for obtaining sectional views. However, such rotating mirror technology is distinctly different from the ranging application disclosed herein.
U.S. Pat. No. 5,350,377 issued Sep. 27, 1994 to Winston et al. teaches a medical catheter using optical fibers that transmit both laser energy and ultrasound imaging signals. An external transducer couples to the optical fibers and pulse echoes are received and transmitted back to the transducer along the same optical fibers. Visualization is limited to images as to the configuration, location and character of the tissue in the area of atherosclerotic plaques.
As is evident by a review of the ultrasound imaging prior art, including the foregoing, catheters and other tools for TMR having axial ranging capability, in the sense of determination of the distance from the tip of the firing laser delivery means at a first wall of the heart to a second wall of the heart are virtually unknown. Determination of tissue depth viewed in a forward direction, such as in myocardial tissue for forming TMR channels, would be highly advantageous so as to prevent unwanted perforation of a heart wall and/or to form channels of selected depths.